For most types of mobile communication system to operate satisfactorily, it is required that the receiver is locked in time and frequency to the transmitter. Traditionally, the receiver achieve-frequency synchronism with the transmitter by controlling the frequency of a local oscillator used to down-convert the signal from RF to base-band (or IF depending on the radio architecture).
FIG. 1 depicts various processing stages that form part of such an approach. Indeed, FIG. 1 can be taken to represent a view into the signal processing chain of a mobile telephone or a cellular communications network base station. It should be noted that the blocks shown in FIG. 1 represent processing operations performed on a received signal but do not necessarily correspond directly to physical units that may appear within a practical implementation of a receiver. The first stage 101 corresponds to the radio frequency processing. During the radio frequency processing, the received signal is down-converted to base-band using a mixer 103. The reference frequency used to drive the mixer is generated by an oscillator 104. Following this carrier down-conversion, the signal is low-pass filtered 102 and then passed to the mixed-signal processing stage 108. The mixed signal processing includes Analogue-to-Digital Conversion (ADC) 105, sampling 106 and low pass filtering 107. The resulting signal, which is now digital, is supplied to the digital signal processing stage 109 where it is buffered in preparation for processing. The demodulation stage 110 produces estimates of the transmitted information bits. As part of the digital signal processing, estimates of the residual frequency offset in the digital signal are produced. Those frequency error estimates are filtered at 111 in order to improve their accuracy and used to control the frequency reference produced by the oscillator 104.
This frequency locking mechanism is typical of mobile communication receivers and achieves synchronism through a feedback loop. Such an approach is very effective in conditions where the frequency reference of the transmitter is stable over time. However, such stability cannot always be achieved. For example in cellular communication systems, the hand-over of a user between different base-stations will result in a short term offset in the frequency of the received signal. This frequency offset will usually be relatively low (0.1 parts per million is a typical value) but can negatively impact on the performance of the demodulation of the signal within the receiver, especially when a high order modulation scheme is used. For example, the performance of the 8PSK modulation used by the E-GPRS (Enhanced General Packet Radio Service) system will be affected by such a small residual frequency offset. In order to limit the performance degradation that such a residual frequency offset causes to the information link, a correction of the receiver frequency reference should be made as quickly as possible. The mechanism described in FIG. 1 is therefore not suitable since the feedback loop introduces a delay in the correction made to the signal.
One possible mechanism to correct this residual frequency offset is depicted in FIG. 2. It can be seen that this approach is similar to the one presented in FIG. 1. However, an extra processing stage has been introduced in the digital signal processing section 209. The digital signal produced by the mixed-signal processing stage 208 is buffered and first processed by the ‘estimate and correct frequency error’ unit 211. The resulting signal, from which the residual frequency offset will have ideally been removed, is then demodulated 210. The ‘estimate and correct frequency error’ unit uses the buffered received signal to first estimate the residual frequency offset. Once this frequency offset has been estimated, a phase correction to the received signal is performed in order to remove it. The residual frequency offset in the received signal can be estimated using frequency component analysis techniques. However, because the residual frequency offset is low compared to the sampling frequency, a large number of samples is usually required in order to obtain an accurate estimate.